1. Field of the Invention
The present invention relates to novel oxide catalysts, methods of producing the same, and methods of producing hydrogen gas by simultaneously performing partial oxidation and steam reforming of methanol (this reaction is called oxidative steam reforming of methanol), and in further detail, relates to novel oxide catalysts, methods of producing these catalysts from a hydrotalcite-like layered double hydroxide and the like, and methods of producing hydrogen gas with a high conversion rate and selectivity using these catalysts, each of with which it is possible to produce hydrogen gas containing no CO whatsoever, or containing only very little CO, by simultaneously promoting partial oxidation and steam reforming of methanol.
The present invention provides novel oxide catalysts, methods of producing these catalysts from a hydrotalcite-like layered double hydroxide and the like, and methods of producing hydrogen gas with a high conversion rate and selectivity using these catalysts.
2. Description of the Related Art
Hydrogen is receiving attention as a new energy source that will replace fossil fuels today when there is a fear of exhausting fossil fuels. Hydrogen is the fuel in fuel cells and is replacing electrical energy. In this case, it is a clean energy source because the only waste product after generation of electricity is water, as well as in terms of measures against global warming. Moreover, hydrogen is an energy source that favors the environment because nitrogen oxides, sulfur compounds, hydrocarbons, etc., which are a burden to the environment, are not emitted. There are two systems of fuel cells, fixed systems that are large and have high output and mobile systems that are small and lightweight, but the fuel cells being studied for use in automobiles, etc., are the latter mobile systems.
The problem here is how to obtain hydrogen. One solution is the method whereby hydrogen is obtained from methanol by any of the following reactions using a catalyst:    (1) Decomposition of methanol            CH3OH2H2+CO ΔH=+92.0 kJ/mol            (2) Steam reforming of methanol            CH3OH+H2O3H2+CO2ΔH=+49.4 kJ/mol            (3) Partial oxidation of methanol            CH3OH+(½)O22H2+CO2ΔH=−192.2 kJ/mol ΔH=heat of the reaction        
The above-mentioned reactions have a problem in that if the gas contains even a trace (20 ppm) of CO when the hydrogen that has been obtained is introduced to a fuel cell and converted to electricity, this CO will damage the Pt electrodes of the fuel cells and there will be a drastic reduction in output. Consequently, it is desirable that the hydrogen gas contain no CO. Nevertheless, by means of above-mentioned reaction (1), large amounts of CO are produced with the hydrogen and by means of above-mentioned reaction (2) as well, although not as much as by reaction (1), 100 ppm or more of CO are produced. Moreover, several 10 ppm CO are produced by above-mentioned reaction (3) and the hydrogen cannot be used as the fuel for fuel cells as is.
Steam is introduced and a water-gas shift reaction (WGSR), or oxidation, is performed in order to eliminate CO from hydrogen gas, but this is accompanied by a new problem in that the device is larger and cost is higher. The reaction formulas of the water-gas shift reaction and oxidation reaction are shown below:    (4) CO Water-Gas Shift Reaction            CO+H2OH2+CO2             (5) CO oxidation reaction            CO+(½)O2CO2         
Consequently, a method of producing hydrogen with which CO is not generated by development of a new catalyst is forthcoming.
The method whereby a CuOZnO catalyst is used is a method of producing hydrogen by partial oxidation of methanol (T. Huang and S. Wang, Appl. Catal., Vol 24, (1986) p. 287).
This method showed highest activity with a Cu:Zn=40:60 catalyst at a reaction temperature of 220 to 290° C. as a result of conducting tests with a Cu:Zn (wt %) ratio=82:18 to 7:93.
However, there was sudden degradation of catalytic activity within an hour. Moreover, there was an increase in the oxygen/methanol ratio as well as an increase in the CO concentration.
Moreover, there are methods that use Cuzn oxide and CuZnAl oxide catalysts (L. Alejo, R. Iago, M. A. Pena and J. L. G. Fierro, Appl. Catal., Vol. 162 (1997) p. 281).
This CuZn-oxide catalyst is Cu:Zn=20:80 to 40:60 and this CuZnAl oxide-catalyst is Cu:Zn:Al=40:55:15.
The mixture of the catalyst precursors Zn5 (CO3)2(OH)6, Cu2(CO3)(OH)2 and Zn3Cu2(CO3)2(OH)6 here becomes a mixture of ZnO and CuO after heating.
The reaction temperature is 200 to 230° C. and the oxygen/methanol ratio is 0.06.
There is further a method that uses Pd-supporting ZnO catalysts (M. L. Cubeiro and J. L. G. Fierro, J. Catal., Vol. 179 (1998) p. 150).
The Pd concentration of this Pd-supporting ZnO catalyst is 1 to 5 wt % and the reaction temperature is 230 to 270° C.
However, an extremely high concentration (20 to 40 mol %) of CO by-product is generated by this method.
Thus, there have been various reports of methods of producing hydrogen by partial oxidation of methanol in the past, but there are problems in that they should be further improved as methods of producing hydrogen without generating CO, and there is a strong demand for development of the same.
On the other hand, there are many reports of methods of producing hydrogen by steam reforming of methanol. However, the water-gas shift reaction or oxidation must also be used in order to eliminate the CO by-product and therefore, a larger device and an higher cost are unavoidable.
Thus, there have been many reports of method of producing hydrogen gas by partial oxidation or steam reforming of methanol in the past, but there are problems in that they should be further improved as methods of producing hydrogen gas with no CO by-product and there is a strong demand for a solution to the same.
Under such conditions, the inventors proceeded with intense studies in order to develop methods of producing hydrogen gas at a high conversion rate and high selectivity by simultaneously performing partial oxidation and steam reforming of methanol in light of the above-mentioned conventional technology and successfully completed the present invention upon (1) discovering methods of preparing oxide catalysts by making catalyst precursors consisting of a hydrotalcite-like layered double hydroxide and the like by coprecipitation and heating these at a temperature of about 450° C., and (2) successfully producing hydrogen gas containing no CO at all, or containing only very little CO, when oxidative steam reforming of methanol was performed using these oxide catalysts.